1. Field of the Invention
The present invention relates to performing downhole operations in wellbores in the field of oil and gas recovery. More particularly, this invention relates to a device adapted to improve the control of the speed of a downhole hydraulic motor.
2. Description of the Related Art
In the oil and gas industry, various operations utilize the rotation of a downhole tool or apparatus. For instance, downhole tools such as drill bits, mills, and scale removal devices are rotated downhole to perform a given operation. A downhole hydraulic motor, such as positive displacement motors (PDMs) and turbines may be used to generate this rotational power.
Generally, a pump at surface injects a working fluid downhole through a drill string, work string, or coiled tubing string. The work fluid is delivered to the downhole hydraulic motor to provide rotational movement to the downhole tool or apparatus attached thereto, such as a drill bit, a scale removal device, etc. For instance, in the case of a turbine, the working fluid rotates the turbine shaft to create rotational movement; in the case of a mud motor, the working fluid rotates the rotor to create rotational movement.
It is known that an optimal, predetermined rate of rotation of a particular downhole tool may be desired (e.g. 400 rpm) to perform a given operation. For instance, it is known to use a scale removal device, such as the ROTO-JET commercially-available from BJ Services Company, to clean scale and debris from a well bore. Such a jetting device is a downhole tool comprised of a set of nozzles mounted to a turbine. Fluid is injected downhole, which spins the turbine shaft within the turbine at a given speed. The fluid passes through the turbine to the jetting device and out the rotating nozzles to remove scale and debris from the wellbore.
It has been discovered that at an optimal rotational speed, the jetting device (having opposing jets aimed in a substantially radial direction) may induce pressure pulsing or stress cycling in the scale that is to be removed from the wellbore. In some instances, the optimum rotation of the jetting device is 400 rpm. Further, by accurately controlling the flow rate of the turbine shaft in the turbine, the life of the turbine is improved.
It is therefore desirable to control the speed of the turbine under varying conditions to optimize de-scaling performance. Thus, it is desirable to have a cleaning jet that rotates at an optimal speed, e.g. 400 rpm, regardless of temperature, injection flow rates, flow rates through the tool, single or two-phase fluid flow, torque loading of the shaft, etc.
Similarly, it is also desirable to improve the control of the rate of rotation of other downhole tools. For instance, optimum life and drilling performance is a significant concern when utilizing a mud motor for drilling or milling, especially with two-phase fluids, as excessive rates of rotation or stalling may occur due to the compressibility of the power fluid. A description of the difficulties associated with the control of mud motors on two-phase fluids is described by Lance Portman, John Ravensbergen, and Paul Salim, in “Controlling Small Positive Displacement Motors when used with Coiled Tubing and Compressible Fluids,” SPE Paper 60756, Copyright 2000, Society of Petroleum Engineers Inc., incorporated herein by reference. Thus, there is a need to improve the control of the rate of rotation of the drill bit by the mud motor, which improves drilling efficiency and increases the mud motor life.
However, in prior art systems, it is not generally possible to maintain the optimal rate of rotation at surface. Generally, the speed of the hydraulic motors is affected by changing the flowrates of the working fluid therethrough. To increase the rotational speed of the downhole hydraulic motor, working fluid flow is increased. However, the actual speed of the hydraulic motor downhole is not known with sufficient accuracy at surface to accurately control the rotation in this way. This is especially true in the case of two-phase (compressible) flow.
Further, many variables impact the output speed of the hydraulic motor: flow rate and pressure drop across hydraulic motor, wellbore temperature, and absolute wellbore pressure. Two-phase flow exacerbates this problem. Thus, it is difficult to sufficiently control the rotational speed of the hydraulic motor and thus of the downhole tool.
It is also known that in prior art systems, it is tedious, difficult, or even impossible to initially set up the tool such that it will operate at a predetermined rate at bottom hole conditions (pressure, temperature, etc.) and for a known or given flow rate. As such, the hydraulic motors may rotate excessively, causing damage to themselves or the tools they are rotating. Alternatively, the hydraulic motors and the downhole tools attached thereto may rotate at a less-than-optimal rate.
Additionally, there are competing demands on flow rate of the circulating or working fluid. For example, the flow rate of nitrogen is typically used to control bottom hole pressure. Cuttings transport is another independent demand on flow rate. In addition, other demands influence the rotational speed generated by the downhole hydraulic motor, such as circulating flow rate, the depth of treatment, well bore temperature, hydrostatic pressure, and frictional pressure drop changes. However well bore conditions are not always known with sufficient certainty, especially bottom hole pressure, to ensure the downhole hydraulic motor rotates at or near the optimal, predetermined level. Therefore it may be difficult to appropriately predict circulating flow rates under the conditions set up for the hydraulic motor and downhole tool, such as a scale removal unit or a drill bit.
Computer modeling may be used to attempt to account for these competing demands on working or circulating flow rate, such that the rate of rotation of the downhole tool is managed and a best compromise can be determined. Further, in some prior art systems, a hydraulic motor is designed in an attempt to rotate at an optimal, predetermined rate, based on various design parameters such as these predicted downhole conditions. However, this has been found to be problematic, since the values of such parameters are not initially known with certainty. Further, the value of these parameters are not constant. Thus, the downhole hydraulic motor rotates above or below the predetermined rate.
Therefore, it is desirable to have an apparatus which may better control the flow rate into the hydraulic motor, such as a turbine, mud motor, etc., such that the rotational speed generated by the hydraulic motor can be controlled across a wide range of flow rates, torque loads, temperatures, pressures, and other operating conditions. This optimizes the performance of the attached downhole tool (e.g. drill bit or de-scaling unit) for drilling or scale removal, for example, and increases in the life of the hydraulic motor and downhole tool attached thereto.
It is also desirable to improve feedback to the operator at surface, especially in the case of two-phase flow.
Thus, a need exists for a device for improving control of the speed of a downhole hydraulic motor. There is a need to regulate the flow rate of the working fluid to the hydraulic motor, such that the rotating element (e.g., rotor or turbine shaft) rotates at an optimal, predetermined rate. The device should take into account changes in the operating conditions—such as temperature, pressure, and flow rates, e.g.—of the downhole tool. It is also desirable for the device to provide improved communication to the operator at surface.
In an attempt to overcome or minimize these problems, one embodiment of the present invention provides two flow paths through the bottom hole assembly: one flow path through the hydraulic motor, which drives the hydraulic motor, and one bypass flow path which is not used to drive the hydraulic motor. The flow control valve of preferred embodiments therefore meters the flow between these two flow paths to control the speed of rotation downhole without surface intervention. The two flow paths may then recombine and enter the downhole tool (such as a de-scaling unit) if desired. Thus, the overall flow rate of working fluid to the downhole tool is not diminished, while the speed of the hydraulic motor is optimized. This is advantageous, for example, when the downhole tool is a de-scaling apparatus having jets, and it is desirous to have as much fluid as possible exiting the jets.